1. Field of the Invention
The present invention relates to an operational amplifier and, particularly, to a circuit protection technique for the same.
2. Description of the Related Art
An operational amplifier is widely used for various purposes as a basic circuit in an electronic circuit. Such an operational amplifier is needed not only in the case where it is integrated as part of an LSI for use but also in many purposes as a single package product mounted on a print board.
A package product of such a circuit part is strongly requested to have reliability against surge, static electricity, and the like. In the case where an operational amplifier is formed as a single package product, an inverting input terminal and a non-inverting input terminal are led out as external electrodes, so that it is necessary to provide a protection circuit for the input terminals.
The inverting and non-inverting input terminals of an operational amplifier are connected to transistors constructing an input differential pair, and have to be protected against an electro static discharge (hereinbelow, abbreviated as ESD) and the like. Protection of an operational amplifier is described in, for example, Patent Document 1.
[Patent Document 1] Japanese Patent Application Laid-Open No. 11-74742.
In the technique described in the above document, a method of improving breakdown voltage by using bipolar transistors as transistors constructing the input differential pair is employed. However, even when bipolar transistors are used, when a high voltage of hundreds V to a few kV is applied across a base and an emitter or across a base and a collector, there is a case that the reliability of the technique is affected.